1. (a) Field of the Invention
The present invention relates to an in-plane switching mode LCD (liquid crystal display) device and, more particularly, to an improvement of such a LCD device for a uniform brightness and a higher image quality.
2. (b) Description of the Related Art
LCD panels used for LCD devices are generally categorized by the mode of the liquid crystal into a plurality of types including a TN-mode (twisted nematic mode) LCD panel wherein the LC layer is driven by a perpendicular electric field, and an in-plane switching mode LCD panel, wherein the LC layer is driven by a lateral electric field.
FIG. 1A shows a TN-mode LCD panel in a sectional view, and FIG. 1B shows the TN-mode LCD panel in a perspective view for illustrating the viewing angle dependency thereof. In the TN-mode LCD panel of FIG. 1A, a perpendicular electric field xe2x80x9cFxe2x80x9d is generated in a LC layer 100 by a set of electrodes. The term xe2x80x9cperpendicularxe2x80x9d as used herein means that the electric field is perpendicular to the surfaces of front and rear panels 101 and 102 sandwiching therebetween the LC layer 100. Each of the panels 101 and 102 mounts thereon an electrode layer for generating the perpendicular electric field.
More specifically, the front panel 101 mounts thereon a common electrode 103 at the interface between the LC layer 100 and the surface of the front panel 101, whereas the rear panel 102 mounts thereon a pixel electrode 104 for each of pixels at the interface between the LC layer 100 and the surface of the rear panel 102.
The common electrode 103 is common to all the pixels of the LCD panel and formed on the entire area of the front panel 101. The pixel electrode 104 is separately disposed for each of the pixels whereby each pixel displays the own image based on the data supplied to the pixel electrode 104. The common electrode 103 and the pixel electrodes 104 are made of transparent substance so that the user observes the image on the LCD panel by way of the light passing through these transparent electrodes 103 and 104.
As shown in FIG. 1B, since the LC molecules 105 in the TN-mode LC layer stands upright with respect to the panels 101 and 102 based on the perpendicular electric field, the image on the LCD panel depends on the direction in which the user observes the LCD panel. That is, the LCD panel has a large viewing angle dependency. For example, the effective viewing angle providing an image having a contrast ratio above 10 is limited to around 30xc2x0 with respect to a perpendicular of the panel as viewed from the upper position, around 20xc2x0 from the lower position and around xc2x145xc2x0 as viewed in the horizontal direction.
FIG. 2 shows an equivalent circuit diagram of the LCD panel shown in FIG. 1A, as viewed from the front thereof. A single pixel 106 includes an equivalent capacitor having a LC layer 100 acting as a capacitor insulator film, and a common electrode 103 and a pixel electrode 104 sandwiching therebetween the LC layer 100, and a thin film MOSFET (TFT) 107 disposed for the pixel electrode 104. The TFT 107 has a source connected to the pixel electrode 104, a drain connected to a corresponding signal line D1 disposed for each column of the pixels, and a gate connected to a scanning line disposed for a row of the pixels.
FIG. 3 shows a schematic overall front view of the LCD panel of FIG. 2, wherein a plurality of the pixels 106 shown in FIG. 2 are arranged in a matrix on the screen of the LCD panel 108. A plurality of scanning lines G1, G2, . . . are disposed for respective rows of the pixels 106, whereas a plurality of data lines D, D2, . . . are disposed for respective columns of the pixels 106. The terminals of the scanning lines G1, G2, . . . are connected to respective output terminals of a gate driver 109 disposed at the left side of the LCD panel 108, whereas the terminals of the data lines D1, D2, . . . are connected to respective terminals of a data driver 110 disposed at the top of the LCD panel 108. The terminal 103T of the common electrode 103 is connected to a ground line.
The in-plane switching mode LCD panel is now replacing the conventional TN-mode LCD panel due to the smaller viewing angle dependency of the in-plane switching mode LCD panel. FIGS. 4A and 4B show the in-plane switching mode LCD panel, similarly to FIGS. 1A and 1B for the TN-mode LCD panel.
In FIG. 4A, the pixel electrodes 204 and the common electrodes 203 are disposed on the rear panel 204, whereby a lateral electric field F1 is formed between the pixel electrode 204 and the corresponding common electrode 203 in each pixel in a direction substantially parallel to the surfaces of both the panels 201 and 202. Other configurations of the in-plane switching mode LCD panel are similar to those of the TN-mode LCD panel shown in FIGS. 2 and 3.
In FIG. 4B, the LC molecules 205 are rotated by the lateral electric field within a plane parallel to both the panels 201 and 203. This configuration affords a smaller difference in the contrast ratio between different angles of the observation by the user. For example, the effective viewing angle of the in-plane switching mode LCD panel affording a contrast ratio of 10 or more is xc2x170 degrees from the top, bottom, left and right sides in observation.
In the in-plane switching mode LCD panel, the common electrodes, pixel electrodes, common electrode line, scanning lines and data lines are generally made of metallic films disposed on the single rear panel. This necessitates smaller widths or a smaller line pitch for these electrodes and lines for achieving a specific light transmittance ratio (or opening ratio) in each pixel area, compared to the conventional TN-mode LCD panel.
The smaller width raises line resistances to increase the voltage drop along the electrodes and the lines and thereby degrade the uniformity in the brightness among the pixels, especially in a large screen LCD panel. The smaller line pitch for the lines and electrodes increases cross-talks between the pixels. Thus, image quality of the in-plane switching mode LCD panel is degraded.
In view of the above problems in the conventional in-plane switching mode LCD device, it is an object of the present invention to provide an in-plane switching mode LCD panel having less cross-talks, a uniform brightness and a high image quality by reducing and substantially equalizing the voltage drop among the pixels.
The present invention provides an in-plane switching mode LCD panel including a LC layer, and first and second panels sandwiching therebetween the LC layer to define a plurality of pixels arranged in a matrix, the first panel including a plurality of data lines each disposed for a corresponding column of the pixels, a plurality of scanning lines each disposed for a corresponding row of the pixels, a common electrode line disposed for the plurality of pixels, each of the pixels including a pixel electrode, a common electrode connected to the common electrode line, and a TFT having a source connected to the pixel electrode, a drain connected to a corresponding one of the data lines and a gate connected to a corresponding one of the scanning lines, a common terminal connected to an end of the common electrode line, a plurality of data terminals each connected to an end of a corresponding one of the data lines, a plurality of first scanning terminals each connected to one of ends of a corresponding one of the scanning lines, and a plurality of second scanning terminals each connected to the other of ends of the corresponding one of the scanning lines.
In accordance with the in-plane switching mode LCD device of the present invention, the first and second scanning terminals connected to each scanning line reduces the signal delay caused by the parasitic capacitance and parasitic resistance of each the scanning line, whereby the scanning line may have a smaller width and thus a smaller line pitch compared to the conventional in-plane switching mode LCD panel.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.